1. Field of the Invention
This invention relates to a process for producing a rolled thermoplastic resin film or sheet. More particularly it relates to a process for producing a film or sheet having improved mechanical strength in the longitudinal direction and in the lateral direction and whitening on bending (hereinafter abbreviated merely to whitening).
2. Description of the Related Art
When thermoplastic resin films or sheets are oriented by rolling, films or sheets uniaxially oriented in the longitudinal direction are generally formed. However, films or sheets oriented in the longitudinal direction have drawbacks such that the difference between the mechanical strength of the resulting films or sheets in the longitudinal direction and that in the lateral direction is not only large, but also when they are bent in parallel to the longitudinal axis thereof, whitening occurs. Such drawbacks have been overcome by orienting them biaxially in the longitudinal direction and in the lateral direction to thereby improve the strength and elastic modulus thereof while retaining the balance between the strength and elastic modulus in the longitudinal direction and those in the lateral direction. For example, Japanese patent application laid-open No. Sho 62-280014/1987 discloses a process of reciprocating a pair of rolls placed parallel to the longitudinal direction of a material in the lateral direction to roll the material in the lateral direction, followed by continuously rolling the resulting material in the longitudinal direction by means of a pair of rolls placed perpendicularly to the longitudinal direction of the material to obtain a sheet or film having a small anisotropy therein. Further, Japanese patent publication No. Sho 46-7478/1971 discloses a process of biaxially rolling a material in an oblique direction while preventing the material from shifting in the lateral direction at the time of the oblique rolling, by means of a pair of planetary rolls to obtain a film or sheet material having a non-anisotropically oriented configuration and having improved mechanical properties.
According to these processes, however, costs required for manufacture of the device and its maintenance as well as its operation are enormous as compared with those in the case of general rolling devices. Moreover, according to rolling by means of the rolls in the lateral direction disclosed in the above Japanese patent application laid-open No. Sho 62-280014 or that by means of the planatory rolls disclosed in the above Japanese patent publication No. Sho 46-7478, there occurs a drawback that contact of the rolls with the film or sheet is discontinuous; hence in order to make uniform the thickness of the rolled film or sheet in the longitudinal direction, operating conditions of the device such as rolling speed, etc. are very much restricted.
The present inventors have made extensive research in order to overcome these drawbacks. As a result, we have found that when thermoplastic resin materials (hereinafter referred to as resin materials) are rolled in an oblique direction to that of the advancing direction of the materials, if a difference is provided between the distance between both of the ends on one side of a pair of rolls and the distance between both of the ends on the other side of the pair of rolls, it is possible to prevent the resin materials from falling off the rolls, and it is also possible to produce a rolled film or sheet having a uniform thickness thereof in the longitudinal direction, a small difference between the mechanical strength in the longitudinal direction and that in the lateral direction, a product exhibiting no whitening when bent, and at a production cost and operation cost similar to those of conventional rolling devices.